The present invention relates to nuclear magnetic resonance (“NMR”) imaging and, more particularly, to methods and apparatus for imaging the peripheral vasculature.
Initially, NMR imaging systems utilized receiver coils which surrounded the entire sample (for example a human patient) that was to be imaged. These remote coils had the advantage that the sensitivity was, to a first approximation, substantially constant over the entire region being imaged. While this uniformity in sensitivity is not strictly characteristic of such remote coils, the sensitivity is substantially constant to a sufficient degree that most reconstruction techniques assume a constant coil sensitivity. Because of their large size the remote coils suffer from a relative insensitivity to individual spins.
For certain applications, a surface coil is preferable to a remote coil. Surface coils can be made much smaller in geometry than remote coils and for medical diagnostic use can be applied near, on, or inside the body of a patient. This is especially important where attention is being directed to imaging a small region within the patient, rather than an entire anatomical cross section. The use of a surface coil also reduces the noise contribution from electrical losses in the body, with respect to a corresponding remote coil, while maximizing the desired signal. NMR imaging systems thus typically use a small surface coil for localized high-resolution imaging.
A disadvantage of the surface coil however, is its limited field of view. A single surface coil can only effectively image that region of the sample having lateral dimensions comparable to the surface coil diameter. Therefore, the surface coil necessarily restricts the field of view and inevitably leads to a tradeoff between resolution and field of view. The size of the surface coil is constrained by the intrinsic signal to noise ratio of the coil. Generally, larger coils induce greater patient sample losses and therefore have a larger noise component, while smaller coils have lower noise but in turn restrict the field of view to a smaller region.
One technique for extending the field-of-view limitation of a single surface coil is described in U.S. Pat. No. 4,825,162 to Roemer et al. Roemer et al. describes a set of surface coils arrayed with overlapping fields of view. Each of the surface coils is positioned so as to have substantially no interaction with all adjacent surface coils. A different NMR response signal is received at each different one of the surface coils from an associated portion of the sample enclosed within an imaging volume defined by the array. Each different NMR response signal is used to construct a different one of a like plurality of NMR images of the sample, with the plurality of different images then being combined to produce a single composite NMR image. Roemer et al. describes a four-coil array for imaging the human spine.
While an increased number of surface coils may be used to increase the field of view, NMR system scanners typically have a limited number of preamplifier inputs. The number of preamplifier inputs is therefore a design limitation in the design of phased array surface coils. A disadvantage of known phased array surface coils, therefore, is that e surface coil array may include only as many coils as can be directly connected to the preamplifiers of the system scanner.
One technique for constructing images of areas of greater size from the limited field of view of known surface coil combinations is to move the surface us after successive scans. This technique however, requires excessive scan room intervention. That is, after each scan, a technician enters the scan room to physically re-position the coils This may increase examination time and increase the likelihood of a patient rejecting the procedure.
It would be desirable to obtain increased field of view without scan room intervention.
It would also be desirable to have an improved phased array surface coil for providing a large field of view. It is further desirable to utilize a greater number of surface coils in the array.